Comma free codes for fast cell search using tertiary synchronization channel

ABSTRACT

A method of processing data comprises the receiving a frame of data having a predetermined number of time slots ( 502,504,506 ). Each time slot comprises a respective plurality of data symbols ( 520 ). The method further comprises a primary ( 508 ), a secondary ( 510 ) and a tertiary ( 512 ) synchronization code in each said predetermined number of time slots.

This application is a divisional of application Ser. No. 10/606,816,filed Jun. 26, 2003, now U.S. Pat. No. 7,630,408, issued Dec. 8, 2009;

Which was a continuation of application Ser. No. 09/418,907, filed Oct.15, 1999, now U.S. Pat. No. 6,665,277, granted Dec. 16, 2003;

Which claimed priority under 35 U.S.C. §119(e)(1) of provisionalapplication No. 60/104,445, filed Oct. 16, 1998 under U.S.C. §119(e)(1).

FIELD OF THE INVENTION

This invention relates to wideband code division multiple access (WCDMA)for a communication system and more particularly to cell search forWCDMA using primary, secondary and tertiary synchronization codes.

BACKGROUND OF THE INVENTION

Present code division multiple access (CDMA) systems are characterizedby simultaneous transmission of different data signals over a commonchannel by assigning each signal a unique code. This unique code ismatched with a code of a selected receiver to determine the properrecipient of a data signal. These different data signals arrive at thereceiver via multiple paths due to ground clutter and unpredictablesignal reflection. Additive effects of these multiple data signals atthe receiver may result in significant fading or variation in receivedsignal strength. In general, this fading due to multiple data paths maybe diminished by spreading the transmitted energy over a wide bandwidth.This wide bandwidth results in greatly reduced fading compared to narrowband transmission modes such as frequency division multiple access(FDMA) or time division multiple access (TDMA).

New standards are continually emerging for next generation wideband codedivision multiple access (WCDMA) communication systems as described inU.S. Pat. No. 6,345,069, issued Feb. 5, 2002, entitled Simplified CellSearch Scheme for First and Second Stage, and incorporated herein byreference. These WCDMA systems are coherent communications systems withpilot symbol assisted channel estimation schemes. These pilot symbolsare transmitted as quadrature phase shift keyed (QPSK) known data inpredetermined time frames to any receivers within the cell or withinrange. The frames may propagate in a discontinuous transmission (DTX)mode within the cell. For voice traffic, transmission of user dataoccurs when the user speaks, but no data symbol transmission occurs whenthe user is silent. Similarly for packet data, the user data may betransmitted only when packets are ready to be sent. The frames includepilot symbols as well as other control symbols such as transmit powercontrol (TPC) symbols and rate information (RI) symbols. These controlsymbols include multiple bits otherwise known as chips to distinguishthem from data bits. The chip transmission time (T_(C)), therefore, isequal to the symbol time rate (T) divided by the number of chips in thesymbol (N). This number of chips in the symbol is the spreading factor.

A WCDMA base station must broadcast primary or first (FSC) and secondary(SSC) synchronization codes to properly establish communications with amobile receiver. The FSC identifies the slot timing from thetransmitting base station. The SSC further identifies a group of sixteenscrambling codes, one of which is assigned to the transmitting basestation. Referring now to FIG. 1, there is a simplified block diagram ofa circuit of the prior art for generating primary and secondarysynchronization codes. These synchronization codes modulate or spreadthe transmitted signal so that a mobile receiver may identify it.Circuits 102 and 110 each produce a 256 cycle Hadamard sequence at leads103 and 111, respectively. Either a true or a complement of a 16-cyclepseudorandom noise (PN) sequence, however, selectively modulates bothsequences. This 16-cycle PN sequence is preferably a binary Lindnersequence given by Z={1,1,−1,−1,−1,−1,1,−1,1,1,−1,1,1,1,−1,1}. Eachelement of the Lindner sequence is further designated z₁-z₁₆,respectively. Circuit 108 generates a 256-cycle code at lead 109 as aproduct of the Lindner sequence and each element of the sequence. Theresulting PN sequence at lead 109, therefore, has the form{Z,Z,−Z,−Z,−Z,−Z,Z,−Z,Z,Z,−Z,Z,Z,Z,−Z,Z}. Exclusive-OR circuit 112modulates the Hadamard sequence on lead 111 with the PN sequence on lead109, thereby producing a FSC on lead 114. Likewise, exclusive-OR circuit104 modulates the Hadamard sequence on lead 103 with the PN sequence onlead 109, thereby producing an SSC on lead 106.

A WCDMA mobile communication system must initially acquire a signal froma remote base station to establish communications within a cell. Thisinitial acquisition, however, is complicated by the presence of multipleunrelated signals from the base station that are intended for othermobile systems within the cell as well as signals from other basestations. The base station continually transmits a special signal at 16KSPS on a perch channel, much like a beacon, to facilitate this initialacquisition. The perch channel format includes a frame with sixteen timeslots, each having a duration of 0.625 milliseconds. Each time slotincludes four common pilot symbols, four transport channel data symbolsand two synchronization code symbols. These synchronization code symbolsinclude the FSC and SSC symbols transmitted in parallel. Thesesynchronization code symbols are not modulated by the cell-specific longcode, so a mobile receiver can detect the FSC and SSC transmitted by anunknown base station. Proper identification of the FSC and SSC by themobile receiver, therefore, limits the final search to one of sixteenscrambling codes that specifically identify a base station within thecell to a mobile unit.

Referring to FIG. 2, there is a match filter circuit of the prior artfor detecting the FSC and SSC generated by the circuit of FIG. 1. Thecircuit receives the FSC symbol from the transmitter as an input signalIN on lead 200. The signal is periodically sampled in response to aclock signal by serial register 221 at an oversampling rate n. Serialregister 221, therefore, has 15*n stages for storing each successivesample of the input signal IN. Serial register 221 has 16 (N) taps242-246 that produce 16 respective parallel tap signals. A logic circuitincluding 16 XOR circuits (230, 232, 234) receives the respective tapsignals as well as 16 respective PN signals to produce 16 output signals(231, 233, 235). This PN sequence matches the transmitted sequence fromcircuit 108 and is preferably a Lindner sequence. Adder circuit 248receives the 16 output signals and adds them to produce a sequence ofoutput signals at terminal 250 corresponding to the oversampling rate n.

A 16-symbol accumulator circuit 290 receives the sequence of outputsignals on lead 250. The accumulator circuit 290 periodically samplesthe sequence on lead 250 in serial register 291 in response to the clocksignal at the oversampling rate n. Serial register 291, therefore, has240*n stages for storing each successive sample. Serial register 291 has16 taps 250-284 that produce 16 respective parallel tap signals.Inverters 285 invert tap signals corresponding to negative elements ofthe Lindner sequence. Adder circuit 286 receives the 16 output signalsand adds them to produce a match signal MAT at output terminal 288 inresponse to an appropriate FSC or SSC.

Referring now to FIG. 3A, several problems arise with this method of FSCand SSC transmission and detection. During first step or first stageacquisition, the receiver must match the 256-chip FSC on a first perchchannel to identify a base station transmission. In the second stage ofacquisition, the mobile receiver must match the SSC to determine whichof 32 possible groups of 16 synchronization codes (ScC) are beingtransmitted and complete frame synchronization by determining which of16 time slots is the first in the frame. Finally, during third stageacquisition, the receiver must determine which of 16 codes in the codegroup is being transmitted. This detection scheme, therefore, limits themobile receiver to identification of a maximum of 512 base stationscorresponding to the 32 groups of 16 scrambling codes each. With thecurrent proliferation of base stations and mobile receivers, however,this has become an unacceptable limitation. Furthermore, simplyincreasing the number of codes per group or the number of groupsdramatically increases time and complexity of the mobile receiver matchcircuit. For example, the present detection scheme requires matching 32possible codes over 16 possible frames or 512 possible combinations.Thus, an increase to 64 groups would introduce 1024 possiblecombinations to match. Finally, the time and power required to matchsuch an inordinate number of combinations is prohibitive.

Turning now to FIG. 3B, other designs of the prior art have tried toresolve this problem by adding a separate step for frame synchronizationprior to synchronization code group identification. These modificationsto the synchronization scheme also envision an alternative embodimentwherein synchronization code group identification precedes the framesynchronization step (FIG. 3C). This modified synchronization scheme(FIG. 3B) adds a third synchronization code (TSC) dedicated to framesynchronization (FIG. 4). This dedicated code enables the receiver tocomplete frame synchronization prior to synchronization code groupidentification. The TSC is transmitted on a third perch channel at thesame symbol time as the FSC and SSC. The long code or original ScC ismasked during this symbol time, so that none of the FSC, SSC or TSC aremodulated by this long code. The TSC, however, is transmitted only oneight even numbered time slots [0, 2, 4, . . . , 14] of each frame andutilizes the same eight 256-chip codes per frame for each code group.

Several problems with the modified synchronization scheme of FIG. 3B andFIG. 3C render them less than ideal. First, additional power allocatedto the TSC will limit system capacity. Second, a balance of powerbecomes more complex due to TSC codes only on alternate frames. Finally,although SSC matching is simplified, it remains complex and timeconsuming.

SUMMARY OF THE INVENTION

These problems are resolved by a method of processing data comprisingreceiving a frame of data having a predetermined number of time slots,wherein each time slot comprises a respective plurality of data symbols.The method further comprises receiving a primary, a secondary and atertiary synchronization code in each said predetermined number of timeslots.

The present invention reduces circuit complexity of FSC, SSC and TSCidentification. Identification time, processing power and circuitcomplexity are further reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be gained by readingthe subsequent detailed description with reference to the drawingswherein:

FIG. 1 is a simplified block diagram of a circuit of the prior art forgenerating primary and secondary synchronization codes;

FIG. 2 is a block diagram of a match filter circuit of the prior art fordetecting the primary synchronization code of FIG. 1;

FIG. 3A-C are synchronization methods of the prior art;

FIG. 4 is a timing diagram showing a sequence of first, second and thirdsynchronization codes of the prior art;

FIG. 5 is a timing diagram showing a sequence of first, second and thirdsynchronization codes of the present invention;

FIG. 6 is a diagram showing a transmit sequence for secondary andtertiary synchronization codes of the present invention;

FIG. 7 is a flow chart showing a method of synchronization codeidentification of the present invention; and

FIG. 8 is an example of 16 length-16 comma-free codes that may betransmitted as the tertiary synchronization code of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 5, there is a timing diagram showing a sequence offirst, second and third synchronization codes of the present invention.The timing diagram includes a frame of data having a predeterminednumber of time slots 502,504,506. This predetermined number of timeslots preferably includes sixteen time slots in each frame. Each timeslot, for example time slot 502 has a duration of 0.625 milliseconds.The time slot is further subdivided into equal symbol time periods.There are preferably ten symbol time periods in time slot 502. A firstsynchronization code (FSC) 508 is transmitted on a primarysynchronization channel during a first symbol time of the time slot. Asecond synchronization code (SSC) 510 is transmitted on a secondarysynchronization channel during the first symbol time of the time slot. Atertiary synchronization code (TSC) 512 is transmitted on a tertiarysynchronization channel during the first symbol time of the time slot.Transmission of this tertiary synchronization code is accomplished via acircuit as in FIG. 1 having an additional multiplier circuit similar tocircuit 104. This additional multiplier circuit receives thepseudo-noise (PN) code on lead 109 and a selected tertiarysynchronization code and produces a modulated tertiary synchronizationcode. Each of the sixteen secondary and tertiary synchronization codeswithin the frame are preferably different from each other. Sixteen ofthe comma free codes in a frame form a comma free code word. Thesesynchronization codes are preferably sixteen comma free codes taken froma set or alphabet of seventeen 256-chip short codes. This set ofseventeen codes is derived from a (16,2) Reed-Solomon code as is wellknown in the art. Each of the selected sixteen codes corresponds to arespective time slot of the corresponding data frame. The order of thesixteen selected codes provides 256 combinations or comma free codewords, each having a minimum distance of 15. These comma free code wordsare sufficient to uniquely identify one of sixteen groups of sixteenlong codes or scrambling codes transmitted by a base station. Apreferred embodiment of the present invention transmits sixteen commafree code sequences from the set {SC₁, SC₂, . . . , SC₁₇} on thesecondary synchronization channel. An exemplary embodiment of thesesixteen synchronization codes is enumerated in rows of FIG. 8. Thepresent invention optionally transmits comma free code sequences fromthe set {φ, SC₁₈, SC₁₉, . . . , SC₃₄} on the tertiary synchronizationchannel as will be explained in detail.

Turning now to FIG. 6, there is a diagram showing a transmit sequencefor secondary and tertiary synchronization codes of the presentinvention. The first row indicates a transmit sequence for TSC_(i)=φrepresenting a null set of sixteen tertiary synchronization codes. Inthis configuration, the present invention transmits one of sixteen commafree code words on the secondary synchronization channel correspondingto one of sixteen scrambling code groups. Each length-16 comma free codeword identifies a respective scrambling code group. Most wirelessapplications are well suited to this configuration of sixteen groups ofsixteen long codes or 256 total long codes. This corresponds to amaximum of 256 different base stations that may be received by a mobilereceiver. In this configuration, the mobile receiver attempts to matchthe TSC with a match filter circuit as in FIG. 2. The match filter,however, fails to detect a match with the TSC null set and produces alow-level output signal MAT on lead 288. This low-level MAT signal iscompared with a minimum threshold value by a threshold comparatorcircuit to recognize the TSC null set. In the absence of a TSC signalfrom the threshold comparator circuit, the mobile receiver performsframe synchronization and matches the SSC code group during second stageacquisition without the TSC. This is highly advantageous in reducingmatch time and complexity for frame synchronization and SSC code groupidentification. Moreover, when the number of scrambling codes in thesystem is small such as 256, no power is allocated to the TSC, therebyincreasing system capacity.

The limitation of 256 code groups, however, is overly restrictive fordense urban areas. The present invention, therefore, provides forvirtually unlimited additional codes by transmitting N distinct commafree codes on the tertiary synchronization channel, where N is aninteger. This permits transmission of 64 comma free codes on thesecondary synchronization channel without increasing match complexity.Operation of the present invention will be described in detail withreference to the flow chart of FIG. 7. The first step of the acquisitionprocess includes identification of a base station FSC by a mobilereceiver. Next, the mobile receiver must attempt to match one of the Ndistinct code words or sequences on the TSC. If no code word isdetected, the mobile receiver produces a low-level match signal MAT aspreviously described and proceeds along the left branch of the flowchart. Alternatively, if the mobile receiver detects one of the N commafree codes on the tertiary synchronization channel, it proceeds alongthe right branch of the flow chart. The mobile receiver uses the TSCmatch to synchronize the frame of the received signal. The receiver thenuses the TSC code to determine the proper frame offset of the code groupon the SSC. When the TSC is present, for example, one of 64 comma freecodes is transmitted on the secondary synchronization channel. When Nhas a maximum value of four, there are N*64 or 256 possible scramblingcodes. Thus, the mobile receiver uses the TSC to provide both framesynchronization and partial synchronization code group identification.

This two-step code group identification is highly advantageous inreducing synchronization match time and complexity for expandedsynchronization code group sets. When there is no TSC code, the mobilereceiver need only match one of sixteen code groups and one of sixteencodes within the group for sixteen cyclic shifts of time slots within aframe. In this case, the code group match of the SSC provides framesynchronization. This yields a match complexity of 16³ or half thecomplexity of the prior art circuits having thirty-two codes per group.Alternatively, when one of N distinct code words is detected on thetertiary synchronization channel, frame synchronization is completed.

Although the invention has been described in detail with reference toits preferred embodiment, it is to be understood that this descriptionis by way of example only and is not to be construed in a limitingsense. For example, the N comma free codes may be any positive integerthat does not exceed the possible combinations of the comma freealphabet. Moreover, other codes may be readily adapted to the presentinvention to accommodate design variations by one of ordinary skill inthe art having access to the instant specification. For example, commafree codes of the present invention may be developed from a (15,2)Reed-Solomon code for a format with fifteen time slots per frame.

It is to be further understood that numerous changes in the details ofthe embodiments of the invention will be apparent to persons of ordinaryskill in the art having reference to this description. It iscontemplated that such changes and additional embodiments are within thespirit and true scope of the invention as claimed below.

1. A process of determining base station scrambling codes in a WCDMAmobile device comprising: A. receiving in the mobile device first andsecond synchronization codes in parallel on respective first and secondsynchronization channels, the first and second synchronization codesoccurring in the same symbol time in subsequent slots of frames ofreceived signals; B. determining one of the absence and presence ofthird synchronization codes on a third synchronization channel, whendetermining the presence of third synchronization codes, the thirdsynchronization codes occurring in all of the same symbol times as thefirst and second synchronization codes; C. in response to determiningthe absence of third synchronization codes, performing framesynchronization and identifying one group of base station scramblingcodes from the second synchronization codes; and D. in response todetermining the presence of third synchronization codes, performingframe synchronization from the third synchronization codes andidentifying one group of base station scrambling codes from the secondand third synchronization codes.
 2. The process of claim 1 in which thereceiving in the device frames of information includes each frameincluding sixteen slots, each slot including ten symbols, and eachsymbol including 256 chips.
 3. The process of claim 1 in which thecertain symbol time is the first symbol time of each slot.